Method and apparatus for recording seismic waves



arch 20, E945. R. A.` BRODING 2,372,055

METHOD AND APPARATUS FOR RECORDING SEISMIC WAVES Filed June 21, 1941 Patented Mar. i945 METHOD AND APPARATUS FOR, RECORDNG SEISMHC WAVES Application .lune 21, 1941, Serial No. 399,170

9 Claims.

This invention relates generally to a method and apparatus for recording seismic waves and more particularly to a system in which a suitable carrier wave of constant frequency is modulated by signal frequencies corresponding to seismic waves.

Electric seismographs have long been used and many types have been devised for the purpose of `system, such as those resulting from high-line "pick-up.

1 In order to overcome the above enumerated objectionable features. the instant invention is dlrected to a system which employs a relatively high frequency, for lexample 1000 cycles per second or hmher, as a carrier wave to be modulated by the seismic waves, detected and recorded by an appropriate camera.

This invention contemplates a method and system that uses a reactance type geophone, two of whose coils serve as inductance elements in a resonant circuit, an oscillator, amplifier and recorder.

' Another object of this invention resides in the provision of a method of recording seismic waves by modulating in a novel manner a relatively high frequency carrier wave with a signal that corresponds to the seismic waves, eliminating the carrier wave and recording signals that correspond to the seismic Waves.

Still another object of this invention resides in the provision of means for detecting seismic waves; the sensitivity of said means can be increased to any desired degree.

Many further advantages and details of this inventionl will be apparent from the following description of the preferred embodiment thereof. It is to be understood it is intended herein to cover not only the combination of the features herein disclosed, but also to cover separately each of the features herein disclosed insofar as those features are patentable.

In the drawing:

Figure l shows a. circuit diagram of'an electric seismograph according to the instant invention;

Figure 2 is a diagrammatic illustration of a geophone of the type used in the instant invention to detect the sismic waves;

Figure 3 shows a single resonant curve of an inductance-capaclty circuit; and

Figure 4 shows two resonant curves illustrating the manner in which the frequency response of the geophone changes with displacement.

Referring to the drawing in detail, particularly Figures 1 and 2, there are shown three stationiary coils I0, II, and I2, all of which are disposed within the geophone'case 26. Coil I2 by means of the conductors I3. is connected to an oscillator It. Oscillator I0 impresses across the coil I2 a carrier -wave of relatively high constant frequency. Coils l0 and II are inductively coupled to coil I2. Connected in parallel with coils I0 and II are variable condensers I5 and I6 respec-y tively by means of which the circuits including the coils Il) and II can be tuned to a frequency slightly dilerent than the carrier Wave. Connected in parallel with coil I I and condenser i6 is a load resistance Il. In like manner, load resistance I8 is connected in parallel with the condenser I5 and coil I0. Load resistances Il and I8 haveadjacent ends connected together yby means of a conductor Ila.` Half wave rectifiers IS and 20 are connected in series in each circuit with such polarity as to cause the voltage impressed across the input of an amplifier 23 by conductors 2l and 22 to be the difference of the two rectified voltages. Output conductors 2l and 22 are connected to the outer ends of the resistances Il' and I8. Filtering condensers C1 and C2 are connected across load resistances IT and I8 respectively, in order to shunt out the high frequency carrier Wave after demodulation by rectifiers I9 and 2li. The output of the amplifier is passed on to the recorder 213 by means of thc conductors 25.

As shown in Figure 2, the coils I0, II and I2 are fixed to the case 26 of the geophone. Cores 27 and 28 for the coils Iii and II are resiliently mounted within the case by means of leaf springs 29 and t0 and serve as an inertia assembly, which on movement of the case will tend to remain stationary: that is. movement of the case 25 will produce relative movement between the cores 2 and 28 and the xed coils Ill and II. The cores il and 28 are so connected and arranged relative to the coils I0 and Il that as one coil increases in inductance. the other decreases. This action is made use of in the circuit which is sensitive to resonant changes of the two tuned inductancecapacity circuits. As the. inductance of these two coils changes oppositely the resonant points of these two circuit-s change in a reversed manner and there results an output voltage across the flesdezl and i2 thatapoeersiasthe; seoohcnefree output visitare representmstheeismic wares will tc njiency. jl'r'rey csrrierfware is eliminated. fcyg an: hetheidiierence of ltliet'woyoitasescorrespond' g g g g f f electricar 'action in martyreaspects similar to oper-sr g ing to the pointes, .and Bin Figure a gli m119,r:1y? i l l 2 f 1 i 1 :ation of al discriminator; i Thum at the; resting the reverseholdsltrue when the-f inductance of; L l zfrec'uencyy or' itnout modulation of ithcciarrier.; .f E coilk iiif increases; andltnatgofcoil iiiiciecrcasesi,.inj f he@ recunedeurrents. sowing in :the: resistance' gwnichfcase the; output ascites@ rsveresitseif. im l ii'llandl182rroduccfqualiandorpositezvoltases: polarity` l l croS'stheIn.' Hencetheirz algebraicsunr iszero. SInicu'rve VH as' snovvnfisi Figure id li'egrleis'131335. r i 'Tuoi condensers CI' and' C2 by v their li filtering trated rasoiran,tjy curve! for the rnsonant'ir-.l l attimi linSf; that the fbreg'ng 'adam-@ni 0f :ths l l cuits il. +153 and 16s-15: where the Q ofthe, colis l, L voltagestakes placeeven tnoughthe rectiners viti r i willr be substantially .smaller than .previously as; L j 2U respectivciy'conductcurrenton thenega f f sinned.l In this; casethe voltage Lc )utput corre-ky l l i tive andpsitive haii Cities 0f the applied signals spending; to the :difference of. the lvertigini et au., 11assumetheorthstis;tsereucloftne@secure and Bfffinrreurezcwouidbe serreslbsiniisiiv f reactance to -the-'resistancc, ortho' coils iB :and 'li I 1 iro i lessfor-thei where the@ isvery much larger, l f L to bs s iiihihetintlieffest DQSUUH and tuned with i f as Shown; by the: survey@ f f; i i f i f f, f i g 1 i :the oondjensers jli :sind i5? a resonant Icurve :as 1 In fthez practice? of this invention the .output .f shown? inrigurefifwilibe obtained; l nlth Fig, voltage for augiven vilnatiorx can bey greatly .in f

re he? resonantiirequencyisiilustrated gas.; ed by useoi coils hol/ins :very high Qs a invention ka carrier frequency; say-'1600 cycles; isy oueney; g 'rheg use of the. high@ come @15g iiitientir femm/ed ifrmi trlie rreisvnant itfpussibjleg to have sharplyrescnantcircunaanddirectionorthcothenito me middle point Such a straight; line; and thus; avoiding @Second hat :t n of hev :curve between? arbitrarily chosen: 2,30 be zvery; shgrpand the; Sensitivity@ ha gegphgnepointsAf aridBf'= Wilibonsrly lince? about point: wouldbe greatlyl entranced.I f i iii; is obvious; however; that the points; A' and i i e f In; has :been :states @that the; se: sins/ 1t 5 ci the "j cannot be chosen jtoofremotjely from' :omicidio f circuit; snownin Figure 1i can ibegreiatly increased; point C, Letg'thle renuencyioitna carrierwave byiincreasingthe voltsaoutputfofgthe oscillatortwill@ e, l instanceicyleSif 51H6@ voltage routput:isizeru.-y and thiswoltafg yout/puy ments are' identical in the rest becomes amaximumin;thezmaximum positions: l, r oi? .thevibrations fof gthe; geophonc; case 2S; carryf; z V,

ance capacity circuits shown ing with it the coils 'l0 |'I, while theinertia be identical and similar to the curve V'. Thereelements 28 and 21 remain substantially fixed. fore, in the rest position both circuits II-IS and The voltages induced in coils I0 and are obvi- IIJ-I5 in Figure 1 must have identical voltages ously linear functions of the voltage across the and of a value corresponding t0 the DOIlt C' in oscillator coil I2. By increasing the oscillator Figure 3. From the circuit shown in Figure 1, it 5 voltage, these two induced voltages in the coils can be seen that the rectified output voltage l0 and can beincreased at will. In this way, across the leads 2| and 22 is the rectified voltage the sensitivity of this type of geophone can be difference between a voltage induced in coil Il greatly increased, correspondingto the maximum and that induced in the coil I0, because the two so positions of the vibrations of the geophone case separate rectilers are connected in series opposand attached c0115 l0 and Il,

ine. Then with a fixed carrier wave, for example A sensitivity control such as here described canof 1000 cycles per second, the geophone at rest, it not be obtained with the usual various types of is clear that the voltage output across the leads variable reluctance geophones and also cannot be 2| and 22 will be zero, and that the tWO voltages made use of in the dynamic types of ueophones. generated in coils I0 and will be equal and Will u The control described is characteristic of the correspond to that voltagel as represented by C' in reactance type of geophone, either in a bridge Figure 3- Therefore. in the rest DOSitiOn there iS circuit or in the resonant circuits as here deno output volta-ge across the input terminals of scribed, the amplifier 23. Thus it can be seen that the geophone operates Now let the vibrations be such as to cause the -00 over a rather steep portion of the curve between inductance of the coil il to decrease and the points A' and B'. By making the Q of the geoinductance of coil I0 to increase. This will shift phone coils high, the steepness of the resonance the resonant curves of the two circuits in the circuits can be raised. Using such a system the manner disclosed in Fixture 4. The inductanceoutput will be much greater than can be realized capacity circuit comprising the coil Il and the 65 with any of the systems known heretofore. condenser I6 now has its resonant frequency I claim:

nearly equal t0 carrier frequency, raisins its volt- 1. A method of detecting andrecording seismic ase corresponding to the point A in Fieure 42 waves that comprises generating an electrical While thel induCtanCe-Copacity Circuit Comprising carrier current wave of a. frequency higher than coil In and the condenser I5 moves farther away 70 that of the seismic waves to be detected. producfrom the carrier freauency and reducing its voltina modulated carrier waves in two resonance cirane corresoondinev to the point B. The two arcuits that are tuned to a common freouency that rows at the too of the vertical line at 950 cycles is slightly different from thel carrier current ireindicate graphically the two directions in which quency by varying the tuning of the two circuits the resonant curves are displaced. The rectified 'w such that one circuit approaches resonance while aszacec 3 the other departs further from it, individually rectifying said modulated carrier waves by half wave rectiers which pass different half waves in the two circuits, electrically adding the diierence of the rectified signals, eliminating the carrier eld by a carrier current wave of a frequency l0 higher than that of the seismic waves to be detected, modulating the induced carrier current wave in accordance with said seismic waves by changing the induced voltages in the coils of two resonance circuits by changing the inductance in each coil such that one circuit approaches resonance while the other departs further from it, rectifying the induced voltages by half wave rectiflers which pass different half waves in the two circuits, algebraically adding the rectified voltsaid branch including a rectiner and a resistance ages, filtering out the carrier, and amplifying and recording the resultant signal.

3. A method of detecting and recording seismic waves that comprises generating an electrical carrier current wave of a frequency higher than that of the seismic waves to be detected. modulating said carrier wave in accordance with said seismic waves by varying the tuned resonance of two circuits such that one circuit approaches rier current of a frequency higher than thatof the seismic waves to be detected, modulating the induced carrier currents in each of two circuits, whose resonant frequency is slightly below that of the carrier current, coupled to the carrier eld by varying the induced voltages in coils in the said circuits coupled to the carrier field by changing the inductance of each coil in an opposite direction, whereby the resonant frequency of one circuit approaches that of the carrier frequency while the other departs therefrom, rectifying the induced voltages in each resonant circuit by separate half wave rectiflers, algebraically adding the rectiiled voltages, and amplifying and recording the resulting signal.

7. In a system in which a carrier current of a vselected frequency is applied to an input circuit which includes two branches, each tuned by impedance elements t0 a resonant frequency differing somewhat from said selected frequency, each across each of which there is developed a direct ,current potential difference, and an output circuit to which there is applied the algebraic sum of said potential differences, the method of producing electrical signals representative of seismic waves which comprises utilizing said seismic waves simultaneously to change the impedance of each of said branches to tune one of them resonance while the other departs further from towards resonance at said carrier frequency it, rectifying the voltages induced in the two circuits by half wave rectiers which pass different half waves in the two circuits, adding algebraically the two rectified voltages. eliminating the carrier wave, and amplifying and recording the $5 resultant signal.

4, A method of detecting seismic waves which comprises generating an electrical field by a carrier current of a frequency higher than that of and to detune'the other away from resonance at said carrier frequency thereby to increase one and to decrease the other of said potential differences by amounts related to the steepness of the resonant characteristic curve of. each of said branch circuits for the development in said output circuit of a. resultant voltage wave comprising the carrier and positive and negative half cycles whose amplitudes are magnifled representhe seismic waves to be detected, modulating the 40 tations of said seismic waves, ltering out the induced carrier currents in each of two circuits, whose resonant frequency is slightly different from that of the carrier current, coupled to the carrier field by varying the inductance oppositely carrier, and recording the resultant signal.

8. The combination as set forth in claim 7, in which each branch circuit has a high ratio of reactance to resistance and a resonant charin each of said circuits, whereby the resonant freacteristic curve of steep slope on each side of each rectified output voltage to filter out the car- 5 rier' currents, algebraically adding the rectified voltages, and amplifying and recording the re sultlng Signal.

5. A method of detecting seismic waves that resonance, and in which, in the absence of seismic waves each branch circuit functions about midway of the region of said steep slope of its resonant characteristic curve.

9. In a system in which a carrier current of a selected frequency is applied to an input circuit which includes two branches each having impedance elements by means of which each circuit may be tuned, each said branch including a comprises generating an electrical eld by a carrectier and a resistance across which there is rier current of a frequency higher than that of the seismic waves to be detected, modulating the induced carrier currents in each of two circuits, whose resonant frequency is slightly diner-ent developed a direct current potential difference, and an output circuit, the method of detecting minute physical changes which comprises tuning each of said branches to a resonant frequency from that of the carrier current, coupled to the differing somewhat from said selected frequency,

carrier field by varying thevinduced voltages in coils in the said circuits coupled to the carrier field by changing the inductance of each coil in an opposite direction, whereby the resonant frequency of one circuit approaches that of the carrier frequency while the other departs therefrom, rectifying the induced voltages in each resonant circuit by separate half wave rectiers, ltering each rectified output to nlter out the carrier currents, algebraically adding the rectied voltages,

and amplifying and recording the resulting signal.

6. A method of detecting Vseismic waves that comprises genera electrical heid by a ccachanges.

ROBERT A. BRODINLi 

